> Clang 6.0 doesn't support LTO and PGO?

No, it appears not.  And it's not an oversight that we don't use the these options.

As Łukasz points out, for the current macOS installer variants we supply are designed to run on all Mac systems from macOS 10.9 on. To accomplish that safely, we build the Python binaries on macOS 10.9 system to ensure they will be compatible, in other words, we build on the oldest system support and rely on upward compatibility when running on newer systems. The other approach is to build on the newest systems available after adding runtime checks throughout the C code to test for the presence of newer features (i.e. runtime calls that have been added in an operating system release newer than the oldest one support).  While this ("weaklinking") can be a viable option, it's a lot more work to implement initially and then keep updated over each o/s release to avoid segfaults and other failures when users on older systems try to use newer features.  Eventually we would like to fully support weaklinking so that we could provide one installer variant for all supported o/s versions that has all features available at each o/s version, it's not a high priority item at the moment (for example, supporting the upcoming 11.0 Big Sur with Apple Silicon is) and the current practices have worked well for many years.

Keep in mind that the main goal of the python.org macOS installers is to provide a single installable binary that works correctly on a wide-range of macOS releases and hardware.  What we provide today works on all Macs capable of running macOS 10.9 or later.  In particular, it is *not* a goal to provide the most optimized configuration for a particular system.  In general, consider the range of hardware and operating system releases, that's not easy to do. I believe that the intended users for the python.org macOS pythons are (1) beginners (like in a teaching environment where ease of deployment and uniformity is key) and (2) third-party Mac applications developers who want an embeddable Python that will allow their applications to work on multiple levels of macOS. If you are looking for the highest performance for a particular use, like benchmarking, you should look elsewhere - like one of the third-party distributors who specialize in numeric Pythons - or build it yourself on your own system.

So, thanks for the suggestion but we won't be using it now. Sometime in the future, if and when we support weaklinking and/or use newer toolchains across the board we will look at adding and other optimizations.